1. Field of the Invention
The present invention relates to a defect inspection apparatus and a defect inspection method for detecting defects such as surface irregularities or dents etc. on a substrate for use, for example, in a process of manufacturing semiconductor devices or the like. The present invention also relates to a method of inspecting a hole pattern such as a contact hole or the like.
2. Related Background Art
In the manufacturing of a semiconductor device or liquid crystal substrate, various different circuit patterns are formed repeatedly in many layers. A process for forming each circuit pattern generally includes the steps of applying a resist on the surface of a substrate, exposing the resist with a circuit pattern on a reticle or a mask by means of an exposure apparatus, forming a circuit pattern of the resist by development, and then forming parts of a device by etching or a similar process. After the pattern of the resist is formed, inspection is performed to determine whether or not there is something wrong with the pattern.
FIG. 7 schematically shows a conventional inspection apparatus used for the above-described purpose. In this apparatus, a semiconductor wafer 2 placed on a stage 3 is illuminated with illumination light L1 and an image of a substrate formed by diffracted light L2 generated by a repeat pattern (not shown) formed on the semiconductor wafer 2 is picked up by an image pickup element 5. The image is processed by an image processing apparatus 6 and defects on the surface of the substrate are detected based on comparison of the obtained image with an image of a normal substrate. Since the direction in which the diffracted light emerges from the semiconductor wafer 2 varies in accordance with the pitch of the repeat pattern, the stage 3 is tilted appropriately in accordance with that pitch. As an example of such apparatus, there is Japanese Patent Application Laid-Open No. 11-51874.
Although the object to be inspected is the resist pattern formed on the uppermost layer (i.e. the outermost layer) of the semiconductor wafer 2, a part of the light that illuminates the substrate passes through the upper most resist layer to illuminate a pattern formed in an underlying layer. Therefore, the diffracted light generated by the substrate as a whole reflects not only the resist pattern of the uppermost layer but also the pattern of the underlying layer. Consequently, when the influence of the pattern of the underlying layer is significantly large, it constitutes a noise and information on the pattern of the uppermost layer that is to be primarily inspected is relatively decreased, so that a problem of deterioration of the S/N ratio arises. Especially, a hole pattern such as a contact hole pattern, which is fine and has a low pattern density, is susceptible to influence of the underlying layer, since the signal intensity is weak. For that reason, it has been impossible in the past to detect defects of a hole pattern reliably.
When an antireflection layer absorbing light used for the inspection is formed between the uppermost layer and the underlying layer, the above-described problem does not arise. However, it becomes difficult to obtain a three-dimensional information of the pattern formed on the uppermost layer.